Generally known principles of suppressing NOx emissions include the suppression of flame (combustion gas) temperatures and a decrease in retention time of combustion gas at high-temperatures. As such, various technologies are available for decreasing the emission of nitrogen oxides by applying these principles. Various methods have been proposed and put into practical use, for example, two-stage combustion, lean-rich combustion, exhaust gas recirculate combustion, water mixing combustion, steam injection combustion, and flame cooling combustion by a water tube group.
Moreover, NOx sources relatively small in capacity such as water-tube boilers are also required for a further decrease in emission of NOx due to an increasing awareness of environmental problems. In this case, the decrease in NOx generation inevitably entails an increased amount of emitted CO, thus making it difficult to attain a simultaneous decrease in NOx and CO.
A cause of the above problem is that a simultaneous decrease in emission of NOx and CO is technically incompatible. More specifically, when temperatures of combustion gas are abruptly lowered and kept at temperatures of 900° C. or less in an attempt to decrease the emission of NOx to result in an ample generation of CO, the thus generated CO is emitted before oxidization to increase the amount of emitted CO. In other words, temperatures of combustion gas are kept higher in an attempt to decrease the amount of emitted CO, thus resulting in an insufficient suppression of NOx generation.
In order to solve the above problem, the applicant has proposed low NOx and low CO emission technologies for decreasing as much as possible the amount of CO, which is generated in accordance with a decrease in the amount of NOx generation, and also suppressing temperatures of combustion gas so as to attain oxidation of the thus generated CO. The technologies are now commercially feasible (refer to Patent Documents 1 and 2). However, an actual value of emitted NOx remains to be about 25 ppm in the low NOx emission technologies described in Patent Documents 1 and 2.
In order to solve the above problem, the applicant has proposed a low NOx combustion method in which a NOx decreasing step is conducted to suppress temperatures of combustion gas so as to give priority to suppression of NOx generation rather than a decrease in the amount of emitted CO, thereby keeping the value of the thus generated NOx to a predetermined value or lower, and a CO decreasing step is, thereafter, conducted so as to keep the value of CO emitted from the NOx decreasing step to a predetermined value or lower (refer to Patent Documents 3 and 4). The technologies disclosed in Patent Documents 3 and 4 are able to decrease the amount of emitted NOx to a value lower than 10 ppm, but find it difficult to decrease the amount of emitted NOx to a value below 5 ppm. This is due to the fact that combustion characteristics inevitably entail NOx generation at 5 ppm or greater.
Then, in the low NOx emission technologies disclosed in Patent Documents 3 and 4, as shown in FIG. 18, combustion is affected at a high air-ratio combustion region Z1 where the air ratio is 1.38 or greater. In contrast, at a combustion region Z2 where the air ratio is 1.1 or lower (herein after, referred to as “low air ratio”), nitrogen oxides are generated in an increased amount, thus making it difficult to attain a simultaneous decrease in the amount of emitted NOx and CO. There is also posed a difficulty in controlling a stable combustion due to a possible occurrence of backfire where the air ratio is 1 or lower. Therefore, the low air-ratio combustion region Z2 has hardly been hardly subjected to research and development. In FIG. 18, the lines F and E graphically show NOx characteristics and CO characteristics on the primary side of a combustion apparatus of the present invention, and the lines U and J graphically show NOx characteristics and CO characteristics of the combustion apparatus of the present invention. Both of the low NOx emission technologies on the secondary side disclosed in Patent Documents 3 and 4 are in principle those in which a burner is used to effect combustion at the high air-ratio region Z1, thereby suppressing the generation of NOx and removing the thus generated CO through an oxidation catalyst (Patent Documents 3 and 4).
On the other hand, there is a growing demand for operating boilers at a low air ratio not only to attain a greater decrease in emitted NOx but also to save energy.
In view of the above-described circumstances, the inventors of the present application have been engaged in research and development of a combustion method for decreasing the amount of emitted nitrogen oxides to zero as much as possible by use of an oxidation catalyst.
Moreover, the method disclosed in Patent Document 5 is known as that for treating nitrogen oxide-containing gas generated on combustion by a burner.
According to the method for treating exhaust gas described in Patent Document 5, in a first step, a burner is used to effect combustion at an air ratio lower than 1.0 (the amount of combustible air lower than the theoretical amount of air), thereby oxygen is not contained in combustion exhaust gas but unburned combustibles such as CO and HC (hydrocarbons) are contained, and a nitrogen oxide reducing catalyst is used to reduce nitrogen oxides by unburned combustibles, thereby purifying the nitrogen oxides.
Then, in a second step, air is supplied to exhaust gas after purification, thereby purifying the unburned combustibles by using an oxidation catalyst.
The treatment method disclosed in Patent Document 5 is not a method for decreasing carbon monoxide and nitrogen oxides in the presence of oxygen. Further, according to the method described in Patent Document 5, unburned hydrocarbons are emitted in a great amount, thus making it difficult to decrease the concentrations of emitted nitrogen oxides and emitted carbon monoxide by using an oxidation catalyst characterized by a poor efficiency in reducing nitrogen oxides in the presence of hydrocarbons. Still further, in a step of reducing nitrogen oxides, a catalyst is used, which is different from that used in a step of oxidizing unburned combustibles, resulting in a complicated constitution of an apparatus and a subsequent difficulty in maintenance and management of the apparatus.
Further, Patent Document 6 describes a method for purifying nitrogen oxide-containing gas emitted from a gas engine. In the method described in Patent Document 6, a three-way catalyst is used to purify nitrogen oxides and carbon monoxide, which essentially requires the presence of hydrocarbons in gas and is applicable only to gas at a theoretical air ratio in which no excess oxygen is present. Therefore, the treatment method described in Patent Document 6 is not appropriately used in treating combustion gas resulting from a boiler, which occurs on combustion by a burner and contains excess oxygen.
Still further, Patent Document 7 discloses a technology in which an oxidation catalyst is used to reduce nitrogen oxides contained in exhaust gas derived from an incinerator by carbon monoxide. According to the technology described in Patent Document 7, since nitrogen oxides will not be reduced in the presence of oxygen in exhaust gas, fuel is burned at an excessively high concentration (air ratio of less than 1) on primary combustion, by which exhaust gas is kept deprived of oxygen. The technology described in Patent Document 7 is subjected to such restriction that fuel is burned at an excessively high concentration, thus making it difficult to find an application for the combustion apparatus such as a burner-equipped boiler in which oxygen is contained in exhaust gas.    PATENT DOCUMENT 1: Japanese Patent No. 3221582    PATENT DOCUMENT 2: U.S. Pat. No. 5,353,748    PATENT DOCUMENT 3: Japanese unexamined Patent Application, First Publication No. 2004-125378    PATENT DOCUMENT 4: U.S. Pat. No. 6,792,895    PATENT DOCUMENT 5: Japanese unexamined Patent Application, First Publication No. 2001-241619    PATENT DOCUMENT 6: Japanese unexamined Patent Application, First Publication No. 5-38421    PATENT DOCUMENT 7: Japanese unexamined Patent Application, First Publication No. 2003-275543